The present invention relates to fiber-optic communication networks, and more particularly, to optical amplifiers with dispersion compensation for use in optical communications networks.
In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber. Optical amplifiers are used in such networks to amplify and reshape optical signals that have been subject to attenuation and dispersion over multi-kilometer fiber-optic links. A typical amplifier may include erbium-doped fiber amplifier components that are pumped with diode lasers and a mid-stage dispersion compensation module. The erbium-doped fiber amplifier stages increase the strength of the optical signals being transmitted over the fiber-optic links. The mid-stage dispersion compensation module is used to compensate for the effects of chromatic dispersion in the transmission fiber.
There are many fiber spans in a typical network link. For example, a long-haul network link may be approximately 400-600 km in length and an ultra-long-haul network link may be 3000-5000 km in length. Each fiber span is typically 40-120 km in length, so there may be many amplifiers in such links. The spans of transmission fiber generally do not all have the same length. In certain areas of the network it may be necessary to use long fiber spans to surmount geographic obstacles. Shorter links may be used in other areas.
Amplifiers that have removable mid-stage dispersion compensation modules are used to accommodate fiber spans of different lengths. For example, when spans of fiber in the network are 100 km in length, the mid-stage dispersion compensation modules in the amplifiers that are used for such spans are selected to compensate for 100 km of fiber dispersion. Although useful for compensating for the effects chromatic dispersion, the dispersion-compensating fiber in mid-stage dispersion compensation modules introduces attenuation and contributes to the noise figure of the amplifier.
It is an object of the present invention to provide optical amplifier arrangements that compensate for dispersion without introducing excessive noise figure penalties.
This and other objects of the invention are accomplished in accordance with the present invention by providing optical amplifiers having dispersion-compensating fiber that is pumped with an optical source to produce Raman gain. By producing Raman gain in the otherwise lossy dispersion compensation section of the amplifier, the noise figure of the amplifier may be reduced.
Removable modules of dispersion-compensating fiber, which may be separate from the Raman-pumped dispersion-compensating fiber, may be used to adjust the amount of dispersion compensation provided by a given amplifier. This allows field technicians to adjust the amount of dispersion compensation that is produced by a given amplifier to match the dispersion compensation requirements of a particular fiber transmission span.
The Raman pump may be formed using fiber-Bragg-grating-stabilized diode lasers or other suitable pump sources. Two cross-polarized diode lasers may be used for the Raman pump to reduce the dependence of the Raman gain on the polarization of the pump. If desired, the dispersion-compensating fiber may be Raman pumped using a two-pass configuration in which pump light passes through a Faraday rotator to minimize polarization-dependent pump effects and to produce additional gain.
Further features of the invention and its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments.